Quartz-to-metal seal



May 16, 1967 D. D. KERSHAW QUARTZ-TO#META L SEAL 3 Sheets-Sheet 1 FiledSept. 29, 1964 w :mm Ts n Ow w T J1 Q M I /u e H b D. D. KERSHAWQUARTZ-TO-METAL SEAL May 16, 1967 5 Sheets-Sheet 2 Filed Sept. 29, 1964Tnvewtor: DeLmaT D. K rshaw by &

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His A t tor'nea United States Patent 3,320,352 QUARTZ-TO-METAL SEALDelmar l). Kershaw, Cleveland, Ohio, assignor to General ElectricCompany, a corporation of New York Filed Sept. 29, 1964, Ser. No.400,267 6 Claims. (Cl. 17450.63)

This invention relates to quartz-to-metal seals for electric dischargedevices such as high pressure vapor or gas discharge lamps operating athigh currents.

The molybdenum foil pinch seals used in mercury vapor lamps are notpractical for currents much in excess of amperes. High intensity compactsource lamps operate at currents ranging from 50 to several hundredamperes. Such lamps may comprise a thick-walled spherical quartzenvelope having seals in the form of protruding stems supporting theelectrodes, and containing a filling of xenon or a metal vapor. Withsuch lamps, two types of seals have achieved commercial status at thepresent time. The first type is the graded seal wherein the stresses dueto the different rates of expansion of quartz and tungsten are reducedto a safe level by interposing a series of glasses with intermediatecoeflicients of expansion between the quartz envelope and the tungstenconductor. The disadvantage of this type of seal is that it requires askilled worker to make it and its reliability is not alwayssatisfactory. The second type of seal utilizes multiple molybdenum foilsconnected in parallel and generally disposed between a pair ofconcentric quartz tubes sealed together. Since the foils are extremelythin, careful handling by skilled workers is again necessary; also theresulting seals become very bulky in high current designs.

A third form of high current seal is described in Patent2,504,522-Greiner, Quartz-to-metal Seal, and follows the Housekeeperprinciple of separating the sealing portion from the current conductor.The seal is in the form of a stem comprising a heavy current conductoror rod extending through a cup or thimble having a feathered sealingedge embedded in the wall of the stem tube. Among the reasons why theGreiner seal did not find commercial acceptance were the excessive costof machining the molybdenum thimble, the weakness of the thimble soproduced, and the difficulty of uniting the thimble to the tungsten rodin a permanently satisfactory fashion. In my copending application, Ser.No. 400,268, filed of even date herewith, entitled Thimble Seal,assigned to the same assignee as the present invention, I have describedand claimed a molybdenum thimble or cup construction and a method ofmanufacturing same and of uniting it to the tungsten rod conductor whichis practical and low in cost.

An object of the present invention is to improve the effectiveness andreliability of the thimble type seal by choice of proportions and byutilizing to advantage the stresses inherently present in the seal tocounter other stresses resulting from the high internal operatingpressure of the lamp. A further object is to provide a convenient andpractical method of manufacturing the seal.

Briefly stated, one aspect of my invention results from the advantageousutilization of the fact that the metal to quartz interface is incompression on the inside of the thimble whereas it is in tension on theoutside. Consequently a design that seals off the pressurized gas withinthe lamp at the inside interface of the thimble achieves a verysubstantial advantage in seal reliability over one sealing off at theoutside interface. In practice, the advantage is realized by facing theopen end of the thimble towards the lamp envelope or source of gaspressure, such being the reverse of the arrangement adopted by theGreiner patent. Other advantages result from the selection of cupdimensions and proportions of isolating .foils.

For further objects and advantages and for a better understanding of theinvention, attention is now directed to the following description andaccompanying drawings of a preferred embodiment and its manufacture.Features of the invention believed to be novel will be more particularlypointed out in the appended claims.

In the drawings:

FIG. 1 is a side view of a compact source xenon lamp having stems withquartz-to-metal seals embodying the invention.

FIG. 2 is a side view, partly sectioned, of the cup and rod conductorassembly.

FIG. 3 is a side sectional view of a vitreous tube assembly which willform part of the stem.

FIG. 4 is a side sectional view of the vitreous tube assembly assembledwith the cup-rod assembly for fusion together.

FIG. 5 is a View similar to FIG. 4 with the stem elements fusedtogether.

FIG. 6 is a side sectional view of the fused stem elements with theinner end cut away.

FIG. 7 is a side sectional view of the completed stem with capillaryjoined to the inner end.

FIG. 8 is a cross sectional view through the stem along section line 88in FIG. 7.

FIG. 9 is a side sectional view of the stem shown rotated 45 on the stemaxis relative to the view of FIG. 7 and joined to a lamp envelopefragment.

Referring to the drawings and more particularly to FIG. 1, theillustrated lamp 1 is a compact source high pressure gas discharge lamp.It comprises a generally spherical clear quartz envelope 2 havingaligned stems 3, 4 protruding from diametrically opposite sides andprovided with quartz-to-metal seals. The stems 3, 4 are identical instructure and have inlead conductors including tungsten rods 5, 6extending through quartz stem tubes and carrying electrodes at theirinner ends, a cathode 7 and a more massive anode 8. Rods 5, 6 haveflexible pigtail leads 9, 11 attached to their outer ends throughconnectors 10; the flexible leads are used to connect the lamp to powersupply terminals. Protective metal tubes 12, 13 are provided over thestems 3, 4 and serve as bases in mounting and mechanically supportingthe lamp.

Lamps of this kind are described as compact source or short are lampsbecause the arc discharge takes place between the tips of the electrodes7, 8 closely spaced within the quartz envelope 2. The lamp contains afilling of an ionizable gas, preferably xenon at a pressure of severalatmospheres. During operation, a gas pressure of the order of 300 poundsper square inch or 20 atmospheres may be produced in the lamp. The arcis characterized by extremely high brightness over a small area and, inthe case of xenon, a well balanced spectral distribution in the visibleregion and a total spectrum approximating that of solar radiation. Atypical lamp of the kind illustrated having an envelope diameter ofapproximately 3 /2" will have a power consumption of 5,000 watts with anoperating current of amperes at 34.5 volts and the initial output willbe 275,000 lumens.

On account of the large currents which must be carried, the tungstenrods 5', 6 must be of such large diameter that quartz cannot be fuseddirectly thereto in a joint which will remain hermetic under theconditions of operation. To provide such a joint, cups or thimbles 14 ofrefractory metal, such as molybdenum or tungsten, are used to effect theseals in conjunction with the tungsten rods 5, 6. The rods extendthrough the cups and are hermetically joined thereto where they passthrough the closed end. The thimbles have a feathered sealing edge 15and the quartz of the stems 3', 4 is fused to both the inner and outersurfaces of the cup about the dge as will be described more fully below.In accordance with the invention, the open ends 15- of the thimbles facein the direction of the envelope volume and this orientation achieves astronger and more temperatureresistant seal. The quartz parts of thestem surrounding the cups and the rods provide mechanical support forthe rods in order to maintain the relatively heavy electrodes 7, 8 inplace within the envelope 2.

The importance of facing the open end of the thimble towards theenvelope may be understood by considering the stresses developed in avitreous material (glass or quartz) which is sealed to the inside andoutside of a thin-wall metal cylinder such as a molybdenum or tungstenthimble. In the quartz immediately adjacent the metal cylinder, threecomponents of stress must be considered, these components being in theradial (P the tangential (P and the axial (P dimensions. The metal has amuch higher coefficient of expansion than the glass or quartz and theseals are formed at the softening temperature of the quartz which ismuch higher than any temperature subsequently encountered in the use ofthe seal. Therefore as the seal cools towards room temperature the glassor quartz will be placed in compression in the axial and tangentialdirection, and this will be so at both the outer and inner surfaces ofthe metal thimble. The radial stresses however will differ as betweeninside and outside, being in tension on the outside and in compressionon the inside. The situation may be summarized in the following tablewherein the minus sign indicates compression and the plus sign indicatestension.

STRESS IN VI'IREOUS MATERIAL SEALED TO A METAL CYLINDER Axial TangentialRadial Outer Surface Inner Surface The significance of the radialtensile stress at the outer surface simply means that the metal istending to pull or break away from the glass. A homely comparison mightbe made to a 'belt sewn in the one case to the inside, and in the othercase to the outside of a garment. Where the belt is sewn to the inside,the stitching is placed in tension and tends to give way as the belt istightened around the body of the wearer, but this is not so where thebelt is on the outside.

It will be noted that the stresses in the glass or quartz on the insideof the thimble are all of the same sign, namely compressive. This is adesirable condition because glass is much stronger in compression thanunder tension, the ratio being about to 1. A high degree of sealreliability may therefore be expected at the inside sealing surface and,in accordance with the invention, this characteristic is utilized toadvantage by facing the open end of the thimble towards the envelopevolume in which gas must be maintained at a high pressure. At the outersurface of the thimble, the radial tension does cause a tendency for thequartz to break or shale off. However, the envelope volume is sealed offat the inside surface of the thimble and a crack at the outer surfacedoes not cause a leak and so is not important.

The seal construction in accordance with my invention will be mostreadily understood by following the process of manufacture withreference to FIGS. 2 to 7.

In FIG. 2 a suitable molybdenum thimble-tungsten rod assembly isillustrated. The considerations affecting the designs of the thimble arethreefold:

(1) The thimble should have sufficient thickness at the closed end tobraze or weld to the rod conductor passing through it.

(2) The thimble walls should be strong enough to withstand the highoperating pressure of the lamp.

(3) The thimble wall thickness should be as small as possible in orderto obtain a seal with maximum reliability.

The most important consideration in the design is meeting therequirement for sufiicient strength with a sealing edge thin enough toseal satisfactorily. I have found that a relatively long or deep thimbleis much preferable to a short, squat cup or thimble in meeting theserequirements. In general, :a thimble having a ratio of length todiameter not less than 1.75 to 1 is desirable: in the illustratedexample, the length is 1.00" and the diameter is .47", suchcorresponding to a ratio of length to diameter of 2.1 to 1. A longlength of thimble permits a relatively thick closed or domed end 16 witha gradual taper to the thin sealing edge 15. In addition, the relativelythick walls at the domed end will withstand atmospheric pressure withoutdistortion during the initial part of the sealing operation to bedescribed below. By way of example, the illustrated thimble is .020"thick at the domed end and radiused at the corners; the walls taperrapidly in thickness from .010 to .005" close to the corners, andthereafter taper gradually to .0003" at the feathered edge.

The molybdenum thimble is hermetically joined to the tungsten rod by afusion process using molybdenum metal to form the fillet 17. This ismuch preferable to conventional brazing wherein a different metal suchas platinum is used at the junction. For further details on the thimbleand the fusion thereof to the tungsten rod, reference may be made to myaforementioned copending application Thimble Seal.

In fabricating a stem, a vitreous assembly is first prepared asillustrated in FIG. 3. It comprises an outer section of stem tubing 18which is necked down at 19 to smaller diameter tubing 20. A short lengthof intermediate diameter tubing 21 is provided with a flare 22 at theend facing neck 19, which flare is sealed peripherally into the outerstern tubing 18. An inner tube 23 of yet smaller diameter is sealed at24 to the end of the intermediate tubing opposite from the flare, andretroverts through the intermediate tubing to the vicinity of the neck19 where its end is closed at 25. During the forming of the seal, it isdifficult to get sufficient heat to the internal glass parts of theseal. Therefore it is helpful to make the inner tube 23 and theintermediate tube 21 of quartz-like glass such as Vycor which has asoftening point approximately lower than quartz or pure silica. Theouter tube 18 is desirably made of quartz.

Prior to inserting the thimble-rod assembly into the stern assembly, itis provided with three molybdenum isolation foils. These foils may beabout .001 thick and their principal function is to prevent the quartzfrom bonding to the metal except in those areas where a hermetic seal isto be achieved. If a bond were allowed to occur at a place where thecross-section of the metal is too great, the bond would rupture uponcooling and the quartz might crack. At the same time, the foils serve ascushions between the unsealed metal and quartz parts and therebyeliminates excessive play. The outer foil 27 is preformed into acylinder and placed around the thimble star-ting at the closed end andextending in the direction of the open end. The edge 28 of the foil inthe direction of the open end is preferably acid-etched beforehand downto .0003" in order to reduce the stress concentration at this point inthe seal. It is convenient in manufacture to hold the foil in place witha few turns of fine molybdenum wire 29, for instance .002" Mo wire. Theinner isolation foil 30 is preformed into a cylinder with a serrated endturned in at 31. An isolation foil 32 is also provided around theportion of rod 5 within the thimble. The inner end of foil 32 islikewise serrated and is turned out at right angle and overlapped by theserrated end of foil 30. lFoils 30 and 32 prevent the Vycor glass innerand intermediate tubes of the stem assembly from sticking or bonding tothe closed end of the thimble and for a distance along its interiorWall, and also from sticking to the tungsten rod where it passes throughthe thimble.

After the isolation foils have been placed in position, it is desirableto fire the entire metal assembly in wet hydrogen. A suitable way ofdoing this is by the use of a radio frequency heater capable of holdingthe parts at about 1100" C. for 5 minutes. It is desirable to tire inhydrogen only such number of thimble-rod assemblies as can readily beused within an hour.

The thimble-rod assembly is inserted into the stem preassembly which isplaced in a conventional glass lathe including jacobs chucks 33, 34(shown in phantom), along with drive means for rotating the head andtail stock chucks synchronously. A rotary vacuum swivel is provided ateach end to allow either vacuum or suitable gas pressure to be appliedto either end. The work is done with the vitreous assembly revolving inthe glass lathe and heat is applied to the work by means of a pair ofoxyhydrogen burners which direct flames against the work from oppositesides and which can readily be moved along the work. After thethimble-rod assembly is inserted into the stern pre-assembly, it isdesirable to lock it in position before starting to rotate the work;this is done by means of a short piece of glas rod 35 which may betacked in place, as indicated in FIG. 4, by applying heat momentarily tothe outer stem tubing 18. The stem tube preassembly is then mounted inthe right-hand chuck of the glass lathe and a piece of flared tubing 36is mounted in the left chuck. By applying heat as required and movingthe chucks together, tubing 36 is joined at 37 to the left end of thestem pre-assembly as illustrated in FIG. 4. The left side of the stemassembly is connected through tube 36 to a vacuum system. The right sideof the assembly may be connected through tube 20 alternately to a vacuumsystem or to a nitrogen pressure system by means of a suitable valvingarrangement. The left side of the stem assembly is exhausted and thenfilled with nitrogen, the process being repeated four times withtheglass moderately iheated between cycles. This degassing process isnecessary toobtain a seal that does not have excessive metal oxide atthe interface. Seals made with a poor vacuum or excessive water show agreater tendency to shale and are less reliable. After the stem assemblyis degassed, the left side is sealed off (by heat collapsing exhausttube 36) when the vacuum gauge indicates a pressure of approximately 1micron. The right side is left connected to the nitrogen pressure systemset at approximately 2 pounds per square inch gauge pressure.

The basic problem in sealing quartz or quartz like glass to both innerand outer surfaces of the thin-walled molybdenum thimble is that ofobtaining a good internal seal without distorting the metal. Since heatis applied from the outside, the tendency is for the outer vitreousportions to become hotter .and collapse onto the thimble before theinner vitreous portions are heated enough to deform and give support tothe thimble walls. This problem is overcome in accordance with theinvention by the use of a thimble which is long relative to itsdiameter, by a sealing system which applies vacuum on one side of theglass parts and gas pressure (nitrogen) on the opposite side, andfinally by control of the initial heating of the stern assembly. Themolybdenum thimble wall thickness decreases from about .0 at the closedend to about .0003" at the feathered open end. I find that it isdesirable to confine the initial heating of the thimble and vitreousassembly to an area extending approximately 10 millimeters along thethimble from its closed end. In this area, the minimum wall thickness is.003" and assure-s sulficient rigidity to prevent deformation despitethe fact that atmospheric pressure is pressing the quartz of outer tube18 against the thimble walls. After about 3 minutes of heating, theintermediate and inner tubes 21, 23 of quartz-like (Vycor) glass areraised to softening temperature. The combination of vacuum on one sideand nitrogen pressure on the other side causes the glass to conform andseal to the inner metal surfaces as illustrated at 2'1, 23' in FIG. 5.The nitrogen pressure inside the thimble then becomes effective insupporting the thimble walls. The sealing process is continued bydisplacing the burners slowly to the right, that is towards the open endof thimble, resulting in an inner sealing that supports the thin openend of the thimble at the same time as the outer quartz tubing 18 underatmospheric pressure collapses into engagement with it, as shown at 18'.By the use of nitrogen pressure on the open side of the thimble and aslow rate of heating, the tendency for the quality of the seal to beadversely affected by dimensional variations in the glass tubing issubstantially overcome. However if the glass tubing is matched to theinternal dimensions of the thimble, the seal can be made in a muchshorter time.

To complete the manufacture, outer quartz tube 18 and inner quartz glasstube 23 are cut off at line AA beyond the place where the flare ofintermediate tube 21 was originally joined to outer tube 18, asillustrated in FIG. 6. Then a piece of heavy capillary or thick-walledtubing of a length appropriate to the lamp size and electrodes is sealedto the end of tube 18 to complete the seal assembly. As an economymeasure, I prefer to save the piece 18a that was cut off from outer tube18 and use it as a sleeve around a quartz capillary tube 38 of smallerdiameter so that the two parts together make up the desired size. Thisalso facilitates using the vacuum-nitrogen system to prevent oxidationof the metal parts during sealing. In sealing, it is preferable to sealonly sleeve portion 18a to the outer tube 18 to leave a gap between theend 23a of the inner tube portion and the end 39 of capillary tube 38.By so doing, the possibility of stress in the vitreous parts by reasonof the fact that inner and outer vitreous portions may have been sealedtogether at different temperatures is avoided.

The capillary tube is not sealed to the tungsten rod and, duringsubsequent lamp manufacture, it is necessary to exhaust and gas fill thecavity within the interior of the cup. In order that this may be donerapidly, I prefer to use a capillary tube 38 having a passage 40 ofsquare cross-section through it, as shown in FIG. 8. During sealmanufacture, after sleeve 18a has been joined to outer tube 18, vacuumis applied and the outer sleeve is caused to collapse onto capillarytube 38 by running the flame along it. Also the capillary tube isvacuumformed, that is collapsed at least partially, onto the tungstenrod 5 for a short distance near the end in order to provide a rigidsupport for the tungsten rod when the heavy electrode is attachedthereto. An isolation foil 41 is wrapped around the tungsten rod toprevent sticking of the quartz to the rod in the area that isvacuumcollapsed.

In the finished stem, the portion of sleeve 18a projecting to the rightbeyond capillary tube 38, and also the portion of outer tube 18projecting to the left beyond the closed end of the thimble are cut offand discarded. The stem then has the appearance shown in FIG. 7 and isready for attachment of an electrode to the end of the tungsten rod,after which it may be sealed to a quartz envelope in manufacturing alamp. To complete a quartz envelope, the inner end of the stem may beinserted into a necked opening in the envelope and the neck wall is thenheat-collapsed onto the stern as shown at 43 in FIG. 9. Alternatively,the inner end of the stem may be flanged and the flange then fused tothe envelope wall. The lamp is completed by steps well known in the art,including evacuation and introduction of the gas filling, whereupon theenvelope is tipped off at 42.

In FIG. 9, the arrowheads indicate the extent of penetration ofpressurized gas from lamp envelope 2 into the stem. Starting at curvedarrows 45, gas penetrates along rod 5, by isolation foil 41 and alongsquare crosssection passage 40, into the hollow cavity within thethimble, as indicated by curved arrows 46. Then proceeding from curvedarrows 47, gas penetrates under inner quartz tube 23, along the closedend of the thimble, and along intermediate tubing 21' up to the end ofisolation foil 30. At this point the quartz is hermetically sealed tothe inside surface of the thimble and this is the limit of gaspenetration. The quartz at the inner interface is in compression in allthree directions, namely in the axial, tangetial and radial directionsso that a strong reliable seal is achieved.

A hermetic seal also exists between the quartz and the outer surface ofthimble 14 beyond the end of outer isolation foil 27. This serves toback up the seal at the inner surface and to strengthen the assembly. Aspreviously explained, the quartz at the outer interface is in radialtension and therefore is more subject to cracking and shaling. However,because the present seal structure effectively seals off the gaspressure at the inner interface, shaling at the outer interface does notresult in a leak and therefore is not fatal to the lamp. In fact I-haveoperated many lamps with the quartz shaled at the outer interface of thethimble seal with no deleterious effects.

While a certain specific embodiment of the invention has beenillustrated and described in detail, the same is to be considered as anexample of the invention whose scope is to be determined by the appendedclaims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A sealed vitreous electric device having a high pressure resistantstem sealed thereto comprising a vitreous outer stem tube, a rodconductor extending through said tube, an elongated metal thimbleencircling said rod, said rod extending through said thimble and beinghermetically joined thereto at its closed end, said thimble having itsopen end facing in the direction of the inner end of said stem which issealed to said electrical device, the open end of said thimble having afeathered edge sealed to said outer stem tube, and a capillary vitreousportion within said outer stem tube at its inner end having an aperturetherethrough through which said rod extends.

2. A sealed vitreous electric device having a high pressure resistantstem sealed thereto comprising a quartz outer stem tube, a tungsten rodextending through said tube, an elongated molybdenum thimble encirclingsaid rod, said rod extending through said thimble and being hermeticallyjoined thereto at its closed end, said thimble having its open endfacing in the direction of the inner end of said stem which is sealed tosaid electric device, the open end of said thimble having a featherededge sealed to the wall of said outer stem tube, and a capillary quartzportion within said outer stem tube about its inner end having anaperture therethrough through which said rod extends.

3. A sealed vitreous electric device having a high pressure resistantstem sealed thereto comprising a vitreous outer stem tube, a rodconductor extending axially through said tube, an elongated metalthimble encircling said rod, said rod extending through said thimble andbeing hermetically joined thereto at its closed end, said thimble havingits open end facing in the direction of the inner end of said stem whichis sealed to said electric device, the open end of said thimble having afeathered edge, the inner surface of said feathered edge being sealed toa vitreous portion extending from said outer stem tube into said thimbleand the outer surface thereof being sealed to said outer stem tube, anda capillary vitreous portion within said outer stem tube at its innerend having an axial aperture therethrough through which said rodextends.

4. A sealed vitreous electric device having a high pressure resistantstem sealed thereto comprising a quartz outer stem tube, a tungsten rodextending axially through said tube, an elongated molybdenum thimbleencircling said rod, said rod extending through said thimble and beinghermetically joined thereto at its closed end, said thimble having itsopen end facing in the direction of the inner end of said stem which issealed to said electric device, the open end of said thimble having afeathered edge, the inner surface of said feathered edge being sealed toa quartz-like vitreous portion extending from said outer stern tube intosaid thimble and the outer surface thereof being sealed to said outerstem tube, said quartz-like portion having a lower softening temperaturethan said outer stem tube, and a capillary quartz portion within saidouter stem tube at its inner end having an axial aperture through whichsaid rod extends.

5. A sealed vitreous electric device having a high pressure resistantstem sealed thereto comprising a vitreous outer stem tube, a rodconductor extending axially through said tube, said rod having an outerend for connection to a current source and an inner end projecting intosaid sealed device, a metal thimble having its closed end at the outerend of said stem and its open end facing in the direction of the innerend of said stem, said thimble encircling said rod conductor, and saidrod conductor extending through said thimble and being hermeticallyjoined thereto at its closed end, said thimble having a ratio of lengthto diameter in excess of 1.75 to 1 and having a feathered sealing edgeat its open end, a portion of said outer stem tube passing around saidthimble and being bonded to the outside of the feathered edge thereof, avitreous portion extending from said stem tube into said thimble andbeing bonded to the inside of the feathered edge thereof, metal foilcovering the portions of said thimble remote from said open end, and acapillary vitreous tube portion extending through and fused to saidouter stem tube at its inner end and having an axial aperture throughwhich said rod extends, said capillary portion providing mechanicalsupport to the inner end of said rod.

6. A sealed vitreous electric device having a high pressure resistantstern sealed thereto comprising a quartz outer stem tube, a tungsten rodextending axially through said tube, said rod having an outer end forconnection to a current source and an inner end supporting an electrodewithin said sealed device, a molybdenum thimble having its closed end atthe outer end of said stem and its open end facing in the direction ofthe inner end of said stem, said thimble encircling said rod conductor,said rod conductor extending through said thimble and being hermeticallyjoined thereto at its closed end, said thimble having a ratio of lengthto diameter in excess of 1.75 to 1 and having a feathered sealing edgeat its open end, a portion of said outer stem tube passing around saidthimble and being bonded to the outside of the feathered edge thereof, avitreous portion extending from said stem tube into said thimble andbeing bonded to the inside of the feathered edge thereof, metal foilcovering the portions of said thimble remote from said open end andhaving a wall thickness too great for hermetic bonding to quartz, and acapillary quartz tube portion extending through and fused to said outerstem tube at its inner end and having an axial aperture through whichsaid rod extends, said capillary portion providing mechanical support tothe inner end of said rod.

References Cited by the Examiner UNITED STATES PATENTS 1,564,690 12/1925Kruh et al 287189.365 2,103,081 12/1937 Kreift 313-318 X 2,504,5224/1950 Greiner 1745().63

LEWIS H. MYERS, Primary Examiner.

H. W. COLLINS, Assistant Examiner.

1. A SEALED VITREOUS ELECTEC DEVICE HAVING A HIGH PRESSURE RESISTANTSTEM SEALED THERETO COMPRISING A VITREOUS OUTER STEM TUBE, A RODCONDUCTOR EXTENDING THROUGH SAID TUBE, AN ELONGATED METAL THIMBLEENCIRCLING SAID ROD, SAID ROD EXTENDING THROUGH SAID THIMBLE AND BEINGHERMETICALLY JOINED THERETO AT ITS CLOSED END, SAID THIMBLE HAVING ITSOPEN END FACING IN THE DIRECTION OF THE INNER END OF SAID STEM WHICH ISSEALED TO SAID ELECTRICAL DEVICE, THE OPEN END OF SAID THIMBLE HAVING AFEATHERED EDGE SEALED TO SAID OUTER STEM TUBE, AND A CAPILLARY VITREOUSPORTION WITHIN SAID OUTER STEM TUBE AT ITS INNER END HAVING AN APERTURETHERETHROUGH THROUGH WHICH SAID ROD EXTENDS.